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Expect to find disease symptoms in wet year

Photos by Tom J. Bechman soybean plant infected with phytophthora root rot
PHYTOPHTHORA AT WORK: This isolated plant succumbed to phytophthora root rot, agronomist Steve Gauck says. Expect to find some of these plants if you are in a wet area.
Soybean Watch: Here’s why more phytophthora root rot isn’t showing up in this field.

Steve Gauck didn’t walk very far into the Soybean Watch ’21 field on his mid-July visit before he found a dead plant. All the symptoms pointed to phytophthora root rot. Yet he wasn’t ready to sound the alarm.

Gauck is a regional agronomy manager for Beck’s, based near Greensburg, Ind. Beck’s sponsors Soybean Watch ’21.

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“We’ve started to get some calls on this, and I would expect more calls,” Gauck says. “Weather conditions where they’ve been getting lots of rain with heat and humidity lend themselves to diseases like this phase of phytophthora root rot showing up fairly early in the season compared to some other years, when it might show up later.”

The good news for the Soybean Watch ’21 field, Gauck says, is that he does not expect the disease to become a major factor that takes out enough plants to impact yield much, if at all.

“These soybeans had two lines of defense,” he explains. “The seed treatment contained fungicides, which helped control fungi that cause diseases like phytophthora root rot early in the season. That protection runs out.

“However, many varieties have either tolerance or some degree of resistance to phytophthora root rot. That doesn’t mean you won’t see it — we found a plant here and there in this field. But you don’t expect to see many plants affected when the variety had tolerance or resistance.”

Diagnosing the disease

Gauck pulled out his copy of the Purdue University Corn & Soybean Field Guide to confirm his suspicions. It’s not the same as a confirmed diagnosis by sending a sample to the Purdue Plant and Pest Diagnostic Lab, but in this situation, he is confident of his conclusions. The guide notes that the first symptoms at this point in the season would be wilting, followed by yellowing and dying leaves.

“We often see the top of the plant folding over,” Gauck says. “The first plant we found in the field looks very similar to the one pictured in the guide as having phytophthora root rot.”

soybean plant infected with phytophthora root rot

Notice the way this plant curled over at the top and died. That is a good indication phytophthora root rot was involved, agronomist Steve Gauck says.

As the disease progresses on an individual plant, lower stems turn black and then roots decay. Gauck easily pulled the plant from the ground and examined the lower stem and roots. The stem was showing signs of turning black, and the roots were decaying as expected.

“We normally see it later in the season, but one sentence in the guide explains the case this year,” Gauck says. “It says, ‘Soils subject to prolonged wet periods are favorable for the disease.’ It also notes that since the fungus is present in the soil, it can appear anytime during the season.”

If you find phytophthora root rot, but only in a plant here and there, your management system is working, Gauck says. You’re likely using a fungicide seed treatment plus soybeans with some form of tolerance or resistance to the disease. If phytophthora becomes a bigger issue in your fields, you may want to take a closer look at your seed treatment practices and choice of varieties, he concludes.

TAGS: Crop Disease
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